Reflective-type smoke detectors work on the principle of light reflectivity of smoke particulates. An L.E.D. light emanating source is open to the interior of a smoke chamber and a phototransistor is mounted alongside thereof for reception of reflected light from the opposite direction from which it is emanated. Reflection of light from the diode actuates an alarm when the density of a given smoke therein reaches a threshold value. The smoke detection capability of reflective-type smoke detectors thus depends upon the color of the smoke, as well as upon its density. The composition of the burning material from which the smoke is emitted then becomes a question as to when a hazardous fire is present.
Light interference-type smoke detectors work on the principle of interference of a light by smoke particulates. An L.E.D. light source is open to the interior of a smoke chamber and a phototransistor is mounted in line thereof for reception of transmitted light from the same direction from which it is emanated.
The Rohm and Haas XP2 Smoke Test Chamber, recently adopted in ASTM D-2843T, and the National Bureau of Standards Smoke Chamber both use smoke analyzers of the light interference-type for the classification of smoke emissions from combusting materials. The object of these tests is to measure and compare the opacity of smoke emitted by burning specific materials under known conditions. Opacity of smoke is only related to density of smoke and does not involve smoke light-reflective characteristics in the operation of these instruments.
In the NBS Smoke Chamber, the smoke from the burning test material rises through a chamber and intercepts a vertical column of light. Transmissibility of light through the smoke in the chamber and its attenuation thereby is measured and recorded at varying smoke emissions therefrom as air vented thereto is changed. As the venting of air to the burning material increases, smoldering changes to open flame burning and open burning changes to smoke dilution by excess air and the density of the smoke emitted increases and then decreases while at the same time the light transmissibility therethrough becomes a minimum.
A common complaint of both firefighters and the fire insurance underwriters is that "you can't describe every fire." Every fire is different because each has its own heat, flame and ventilation characteristics and each has its own composition of burning materials. All parameters which describe the characteristics of a fire are rarely identical for different fires. For that matter, fire parameters can vary within a fire itself -- especially if the fire is extensive or the air venting changes from place-to-place or time-to-time.
In comparing the smoke emissions of fires, certain parameters which best describe the smoke-emitting characteristics of a particular fire include the availability of oxygen to the fire, the chemical composition and the physical characteristics of the burning material on fire, and the convection, conduction and radiation of heat from the fire. Once all these parameters are known, the opacity of the smoke emitted from the fire is fixed.
Assuming smoke opacity is a sole criterion for the presence of fire, the optical density of smoke emitted therefrom forms the basis for the operation of a fire detector. Optical density of smoke depends upon both smoke particle size and the number of smoke particles in a given volume. Optical density does not depend on the color of the smoke or its light-reflecting characteristics. Smoke detectors that use optical density as a basis of operation then are to be distinguished from those that use light reflectivity for that purpose in that they are sensitive to the color of the smoke. In smoke detectors that operate on a light interference principle, the color of the smoke does not make any difference in the performance characteristics of the unit.
Existing smoke detectors operate on a continuous basis. Continuous smoke detection is not only unnecessary but is not required under existing fire prevention regulations or underwriting codes. Continuous operation of smoke detection equipment cuts short the service life of the detector and also consumes more energy than is absolutely necessary. All of these factors make for high cost of operation and maintainability of existing smoke detection equipment.
There is thus an established need for a smoke detector which works with equal sensitivity on smoke of all colors and which is more cost-effective from operational, maintenance and service-life standpoints.